The Global Navigation Satellite System (GNSS) is an accurate provider of Global Position, Navigation and timing (PNT) information, depending on capable receivers and dense satellite constellations. GNSS users generally assume that the position and timing information provided by their receivers is completely accurate, but for more complex users, their requirements go beyond accuracy and extend to the integrity of GNSS information.
Integrity refers to the capability of a system to provide a timely alarm to users when GNSS signals and the system are unavailable for navigation. A wide-area enhancement system calculates a difference correction number at the central station and provides the users with integrity information. In other words, through the monitoring of GNSS signals, the integrity of GNSS satellites and ionospheric grid points is analyzed and alarm information indicating “do not use” is sent. When the system cannot determine the integrity of the GNSS satellites or ionospheric grid points, it sends information indicating “unmonitored” to the users.
Providing accurate positioning, navigation and timing services is crucial to Air Traffic Management (ATM), especially in civil aviation, which is a major concern for safety. In this sense, SESAR and NextGen have identified the use of GNSS as the primary future navigation system for civil aviation. However, due to the low power level of GNSS signals, SESAR and NextGen may be prone to faults, resulting in disruption of navigation services. In fact, the high threat of radio frequency interference leads to the need for a GNSS backup system designed to provide civil aviation navigation services when the GNSS is unavailable, i.e., an Alternative Position, Navigation and timing (APNT) system. APNT refers to a separate non-GNSS system designed to provide PNT information during the unavailability of the GNSS. The broad recognition of the Global Positioning System (GPS) vulnerability in the Volpe report spurs further exploration of the APNT system, including the development of eLoran in the United States and Europe, general work on ranging signals, DME-DME positioning, and the European R model. The work is intended to combine observable measurements of two systems through loosely or tightly coupled integrated receivers to produce as good PNT information as possible. In fact, since the accuracy of APNT positioning solution is usually lower than that of GNSS positioning solution, the accuracy of the comprehensive positioning result obtained through the above work is almost equal to that of GNSS positioning alone. Nevertheless, due to the unavailability of the GNSS, the APNT services should still be valued, and the goal in the future should be to improve the positioning accuracy of the APNT services.
At present, the following problems exist in the implementation and application of the APNT services under different terrain conditions. The first problem is as follows: for users at medium altitudes, terrain and urban features may cause partial occlusion of the ranging source; although the APNT services may still be implemented normally, their positioning and timing results are not very accurate and may have a large error. The second problem is as follows: for users at low altitudes, the occlusion of the ranging source is more serious. Under such conditions, the implementation of the APNT services is more difficult. For example, when GNSS service is unavailable in a canyon airport, aircraft positioning is a great challenge.
In view of the above two problems, corresponding solutions are given in the existing research. For the first problem, in previous studies, most scholars classify high-altitude users and super-high-altitude users into the same category, and conducts positioning based solely on a limited number of ground ranging sources. Since the ground ranging sources available to the high-altitude users are fewer than those available to the super-high-altitude users, it is not reliable though the positioning solution can be obtained.
For the second problem, with the comprehensive deployment of the GNSS, the idea of air-to-air positioning has been put forward so as to obtain a more accurate positioning solution when the GNSS is unavailable and the available ground ranging sources of the low-altitude users cannot support its positioning. That is, if a high-altitude aircraft has enough available ground ranging measurements, it can calculate the position solution and become an airborne ranging source to assist a low-altitude aircraft in positioning, especially for an aircraft in a canyon airport. Conversely, the high-altitude users cannot take advantage of the low-altitude users to conduct positioning, because the “layered” aspect of A2A positioning means that users have to resort to ranging sources with positioning solutions. Although the air-to-air method can be used for positioning low-altitude users, the accuracy is still quite different compared with the conventional GNSS positioning solution.
Besides, in the study of APNT integrity, a Receiver Autonomous Integrity Monitoring (RAIM) algorithm is mostly introduced into fault detection, which solves a protection level and performs a series of performance evaluation by calculating a fault slope. However, with the development of the GNSS, the conventional RAIM algorithm is no longer suitable for multi-navigation systems, and its application scenarios are not limited to the ocean route stage.